Electronic effects in transition-metal porphyrins. 4 ... - ACS Publications

Feb 4, 1982 - San Francisco State University, San Francisco, California 94132. Electronic Effects in Transition-Metal Porphyrins. 4. Effect of Unsymme...
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Inorg. Chem. 1982, 21, 3342-3348 Contribution from the Department of Chemistry, San Francisco State University, San Francisco, California 941 32

Electronic Effects in Transition-Metal Porphyrins. 4. Effect of Unsymmetrical Phenyl Substitution on the NMR Spectra of a Series of Metal-Free and Zinc Tetraphenylporphyrins F. ANN WALKER,*' VIRGINIA L. BALKE, and GREGORY A. McDERMOTT Received February 4, I982

The series of six metal-free porphyrins having all possible numbers of p(diethy1amino)phenyl and p-chlorophenyl groups, H2TPP@-Cl),(p-NEt2), (x + y = 4; x = 0-4), have been synthesized and separated by chromatographic techniques. Their 250-MHz NMR spectra show patterns of pyrrole-H resonances which are fully consistent with the pattern of para phenyl substitution for each isomer. There are two regions of pyrrole-H resonances: one at 8.74-8.80 ppm and one at 8.92-9.04 ppm, for those protons immediately next to meso positions carrying a p-chlorophenyl or p-(diethy1amino)phenyl group, respectively. The weighted average pyrrole resonance position of the six isomers shifts slightly upfield as the sum of the Hammett u constants of the substituents increases and follows the same linear free energy correlation as do a series of H2(p-R),TPPs (ppyrrolc.H = 4 . 0 172 ppm). However, while the weighted average pyrrole resonance position shifts upfield, the individual pyrrole-H resonances of the four unsymmetrical isomers shift downfield as the sum of the Hammett u constants increases. These ring current shifts and those of the ortho phenyl protons are interpreted in terms of the electronic effects of phenyl substituents being transmitted to the porphyrin a system by a combination of u and a induction rather than u induction and a conjugation.

Introduction The degree and mode of transmission of electron density to and from various points on the porphyrin ring through the 0- and ?r-orbital systems, the effects of changes in electron density upon the basicity of the four porphyrin nitrogens, and the reactivity and spectroscopic properties of the central metal ion, if present, have long been interests of those who have investigated the physical properties and chemical reactions of metall~porphyrins.*-*~Early studies, mainly by Caughey and

Recipient, NIH Research Career Development Award, 1976-198 1. Falk, J. E. "Porphyrins and Metalloporphyrins"; Elsevier: New York, 1964; DD 28. 42. 69. 93. Caughe), W. S.; Deal, R. M.; McLees, B. D.; Alben, J. 0. J . Am. Chem. SOC.1962,84, 1735-1736. Caughey, W. S.; Fujimoto, W. Y.; Johnson, B. P. Biochemistry 1966, 5 , 3830-3843. McLees. B. D.; Caughey, W. S. Biochemistry 1968, 7, 642-652. Baker, E. W.; Storm;C.-B.; McGrew, G. T.; Corwin, A. H., Bioinorg. Chem. 1973, 3, 49-60. Higginbotham, E.; Hambright, P. Inorg. Nucl. Chem. Letr. 1972, 8, 747-750. Worthington, P.; Hambright, P.; Williams, R. F. X.;Feldman, M. R.; Smith, K. M.; Langry, K. C. Inorg. Nucl. Chem. Lett. 1980, 16, 441-447. Williams, G. N.; Hambright, P. Inorg. Chem. 1978, 17, 2687-2688. Kirksey, C. H.; Hambright, P.; Storm, C. B. Inorg. Chem. 1969, 8, 2 141-2 144. Walker, F. A.; Hui, E.; Walker, J. M. J . Am. Chem. SOC.1975, 97, 2390-2397. Walker, F. A.; Beroiz, D.; Kadish, K. M. J . Am. Chem. SOC.1976, 98, 3484-3489. Walker, F. A,; Io, M.-W.; Ree, M. T. J . Am. Chem. SOC.1976, 98, 5552-5560. Vogel, G. C.; Beckmann, B. A. Inorg. Chem. 1976, 15, 483-484. Jones, R. D.; Summerville, D. A,; Basolo, F. J. Am. Chem. SOC.1978, 100, 4416-4424. Walker, F. A.; Barry, J. A.; Balke, V. L.; McDermott, G. A,; Wu, M. 2.; Linde, P. F. Adu. Chem. Ser., in press. Kadish, K. M.; Morrison, M. M. Bioinorg. Chem. 1977,7, 107-115 and references therein. Kadish, K. M.; Bottomley, L. A. J. Am. Chem. SOC.1977, 99, 2380-2382. Kadish, K. M.; Bottomley, L. A.; Beroiz, D. Inorg. Chem. 1978, 17, 1124-1129. Rillema, D. P.; Nagle, J. K.; Barringer, L. F.; Meyer, T. J. J. Am. Chem. SOC.1981, 103, 56-62. Shears, B.; Shah, B.; Hambright, P. J. Am. Chem. SOC. 1971, 93, 716-778. Reid, J. B.; Hambright, P. Inorg. Chem. 1977. 16, 968-969

0020- 166918211321-3342$01.25/0

c ~ - w o r k e r s , ~ -of~ ,such * ~ effects in the natural porphyrins and their metal complexes provided valuable information concerning the relative basicities of the free-base forms and the axial ligation properties of natural and modified natural porphyrins. Chemically modified derivatives of protoporphyrin IX complexes of iron have recently been extremely valuable in probing the active sites of various heme proteins by N M R s p e c t r o s ~ o p yin, ~part ~ ~ because ~~ they produce varying degrees of asymmetry in the n-electron system of the porphyrin which are revealed by the pattern of unpaired electron delocalization of the low-spin iron(II1) forms of these proteins. Chemical modifications of protoporphyrin IX are typically made a t the 2- and/or 4-pyrrole positions (Figure la). Thus substituents can interact directly with the n-electron system of the porphyrin ring, and the separation af resonance and inductive contributions to linear free energy relationships for natural porphyrins and their chemically modified relatives has not been possible. Furthermore, since naturally occurring porphyrins and their chemically modified derivatives are, by nature's design, of low molecular symmetry, it is difficult to quantitate the effect of substituents on orbital energies and the inherent symmetry of electron distribution in the porphyrin ring. In the years since the original studies of Caughey and coworkers on the effects of substituents on the physical and chemical properties of natural porphyrins, a number of studies have been reported on related synthetic porphyrins, in particular tetraphenylporphyrins and their metal c o m p l e ~ e s . ~ - ~ ~ These meso-substituted porphyrins (Figure 1b) are a totally synthetic family of molecules, which, although they cannot be substituted for heme in the proteins, have the advantage of allowing the systematic and quantitative study of the physical and chemical properties of the porphyrin nucleus because of the wide range of possible substituents that may be introduced onto the phenyl rings. They have the further advantage of high molecular symmetry, which simplifies the observed N M R spectra and the interpretation (in terms of (23) Meot-Ner, M.; Adler, A. D. J. Am. Chem. SOC.1972, 94,4763-4764. (24) Meot-Ner, M.; Adler, A. D. J. Am. Chem. SOC.1975,97, 5107-5111. (25) Wiithrich, K.; Shulman, R. G.; Wyluda, B. J.; Caughey, W. S. Proc. Natl. Acad. Sci. U.S.A. 1969, 62, 636-643. (26) LaMar, G. N. In "Biological Applications of Magnetic Resonance"; Schulman, R. G., Ed.; Academic Press: New York, 1979; pp 305-343 and references therein.

0 1982 American Chemical Society

Electronic Effects in Transition-Metal Porphyrins

Inorganic Chemistry, Vol. 21, No. 9, 1982 3343

R

Y

R

a

b

C

Figure 1. (a) Structure of protoporphyrin IX (R, = R2 = vinyl) and its chemically modified derivatives (Rl, Rz= H, CH2CH3,CHO, Br, etc.). (b) Structure of symmetrical tetraphenylporphyrins. (c) Structures of the six HzTPP(p-Cl),(p-NEt2), isomers of this study: 1, X1-X4 = C1; 2, Xi = NEt2, X2-X4 = C1; 3; Xi, X3 = NEtz, X2, X4 = C1; 4, Xi, Xz = NEt2, X3, X4 = C1; 5, Xi, Xz, X4 = NEtz, X3 = Cl; 6, X1-X4 = NEt,. Note that the meso positions are labeled CY, p, 7,6 to aid discussion in the text.

molecular orbital theory) of UV-visible, N M R , and other types of spectra which have therefore been investigated in detail. Such systematic investigations of these highly symmetrical compounds have allowed extraction of quantitative information such as the chemical shifts of pyrrole and N-H proton^,^^,^* rates of phenyl r ~ t a t i o n , ~ porphyrin *-~~ inverion,^^,^' N-H proton t a u t o m e r i ~ m ,and ~ ~ ~evidence ~~ of atropisomerism of ortho-substituted derivative^.^^ Linear free energy relationships have been found for rate9 and equilibrium1&16constants of axial ligand addition, including both Lewis base1&16and molecular o ~ y g e n ' ~addition J~ reactions, redox potentials12.1620for both porphyrin ring and metal redox reactions, metal incorporation r a t e ~ , ~ the l - ~ energies ~ of electronic absorption^,^^ etc. In almost all cases (except for ref 23, 24, and 30, as discussed below) the above quantities have correlated with the simple u constants of the Hammett equati~n~~-~'

Px = 4uxp

(1)

where Px is the observed physical property of a compound having substituent X, ux is the substituent constant of X as defined by the relative dissociation constants of a series of X-substituted benzoic a ~ i d s ,and ~ ~p!describes ~~ the sensitivity of the property under study to the electron-withdrawing or -donating properties of X. The question has often been raised as to whether the transfer of these electronic effects from meta- or para-)< groups on the phenyl rings of TPP to the porphyrin ring is by inductive or resonance transfer or a combination of the two. Though most free energy relationships have related Px to the original Hammett u constants (eq l), the rates of metal incorporation into the free-base porphyrins23 and electronic absorption maxima of the totally protonated free-base porphyrins (known as the d i ~ a t i o n s have ) ~ ~ been found to correlate with u+ 37 or Janson, T. R.; Katz, J. J. In 'The Porphyrins"; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. IV, pp 1-59 and references therein. LaMar, G. N.; Walker, F. A . In 'The Porphyrins"; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. IV, pp 61-157 and references therein. Eaton, S. S.; Fishwild, D. M.; Eaton, G. R. Inorg. Chem. 1978, 27, 1542-1 545. Eaton, S. S.; Eaton, G. R. J . Am. Chem. SOC.1977,99,6594-6599 and references therein. LaMar, G. N.; Satterlee, J. D.; Snyder, R. V. J . Am. Chem. SOC.1974, 96, 7137-7138. Snyder, R. V.; LaMar, G. N. Ibid. 1976, 98, 44 19-4424. Storm, C. B.; Teklu, Y . J . Am. Chem. SOC.1972, 94, 1745-1747. Eaton, S. S.; Eaton, G. R. J . Am. Chem. SOC.1977, 99, 1601-1604. Walker, F. A.; Avery, G. L. Tetrahedron Lett. 1971, 4949-4952. Hammett, L. P. Trans. Faraday SOC.1938, 34, 156-165. Leffler, J. E.; Grunwald, E. "Rates and Equilibria of Organic Reactions"; Wiley: New York, 1963; pp 172, 173. Swain, C. G.; Lupton, E. C. J . Am. Chem. SOC.1968,90,4328-4337.

u - u+, indicating strong resonance coupling between phenyl and porphyrin rings. Rates of porphyrin inversion have been shown to be a nonlinear function of the original Hammett u constants when the p(diethy1amino)phenyl substituent was included in the c ~ r r e l a t i o n Our . ~ ~ own studies16of this latter substituent, however, have shown it to behave linearly with the Hammett u constants for a number of "ground-state" or equilibrium properties such as reactions involving ligand addition to metalloporphyrins,16 electrochemical redox potentials,16and, as summarized herein, N M R shifts of the various porphyrin protons, as well as proton isotropic shifts of low-spin iron(II1) p o r p h y r i r ~ s ~and * , ~ the ~ g anisotropy measured from EPR parameter^,'^,^^ discussed elsewhere. Recent studies of photoisomerization of the "picket-fence porphyrin" have suggested that electronic excited states of porphyrin free bases may indeed have stronger resonance coupling between phenyl and porphyrin rings than do ground states.41 This may account for the anomalies noted for activation energies of phenyl rotation, metal incorporation, and electronic spectra of the dications, or it may simply be that in each of these cases, but not others, the porphyrin ring is highly distorted from planarity, thus making direct resonance coupling possible. For tetraphenylporphyrins in their ground electronic states, molecular models (CPK) suggest that because of steric interference between the &pyrrole and ortho phenyl protons, the phenyl rings are constrained to being out of the plane of the porphyrin most of the time, and X-ray crystallographic structure determinations have confirmed this suggestion in numerous cases.4z The barriers to rotation of the phenyl rings in Ru(III), In(III), and Ti(1V) TPPs have been determined by N M R techniques and are found to range from 11 to 17 k~al/mol,~O relatively high energy barriers. These facts have suggested that the transfer of electronic effects from phenyl to porphyrin, and its consequent effect on "ground-state" (equilibrium) properties must be largely inductive. However, theoretical calculations of W01berg~~ have suggested that in solution the phenyl rings can achieve positions nearly coplanar with the porphyrin ring and thereby allow significant resonance transfer of substituent effects. Such phenyl-porphyrin r-orbital overlap may explain kinetic result^.^^*^^ The question remains as to the importance of resonance to the equilibrium properties of metalloporphyrins and their free bases. In a recent study of the effects of substituents on the equilibrium constants for complexation of piperidine to a series of V(1V) and Ni(I1) porphyrins" it was found that there was evidence of almost equal contribution of resonance and ( u ) inductive transfer,l' although it was difficult to distinguish clearly between the possibility that the resonance portion of the transfer of electron-donating or -withdrawing effects of substituents from the para phenyl position to the metal center is by P induction or by moderate P conjugation as defined by Taft and co-workers."'' Because of the inability to distinguish between r induction and moderate P conjugation in the Ni(11)-substituted TPPs, we have sought other systems which would allow such a distinction to be made. We have recently prepared the series of unsymmetrically phenyl-substituted tetraphenylporphyrins shown in Figure IC, which provide Walker, F. A.; Balke, V. L.; McDermott, G. A . J . Am. Chem. SOC., 1982, 104, 1569-1574. Walker, F. A.; Benson, M., submitted for publication. Walker, F. A.; Reis, D.; Balke, V. L., submitted for publication. Freitag, R. A.; Mercer-Smith, J. A.; Whitter, D. G. J . Am. Chem. SOC. 1981, 103, 1226-1228. Meyer, E. F.; Cullen, D. L. In "The Porphyrins"; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. 111, pp 513-529 and references therein. Wolberg, A. J . Mol. Struct. 1974, 21, 61-66. Dayal, S. K.; Ehrenson, S.; Taft, R.W. J . Am. Chem. SOC.1972, 94, 9113-9122.

Walker, Balke, and McDermott

3344 Inorganic Chemistry, Vol. 21, No. 9, 1982

a

b

Rgure 2. Electron density and nodal properties of the porphyrin 4e(~) orbitals$6 which are the LUMOs of metal-free porphyrins and met a l l o p o r p h y r i n ~ . ~The ~ ~ ~light ~ and dark shades represent orbital symmetry properties. The sizes of the circles depict the relative sizes of the squares of the atomic orbital mixing coefficients, ci2,for each atom and thus represent the relative electron density expected at that position. The 4e(r) orbitals shown in a and b are linear combinations of those shown in ref 46, which are appropriate for meso-substituted porphyrins. Compounds 2 and 3 are expected to utilize orbital a for resonance delocalization of para phenyl substituents while 5 is expected to utilize orbital b.

N M R evidence that the “resonancen contribution is that of T induction. The reason that NMR spectroscopy should provide evidence for a conjugation in compounds such as those of Figure IC, especially for compounds 2 and 5, where three phenyl substituents are the same, may be understood if one considers the porphyrin a orbitals which are expected to be used for such a conjugation. The orbitals expected to be involved are the HOMO (for electron transfer from porphyrin to phenyl) and the LUMO (for electron transfer from phenyl to porphyrin). The HOMO is expected to be either AZuor AI, (typically the f ~ r m e r )and ~ ~in, ~either case a nondegenerate orbital of 4-fold ~ y m m e t r y , whereas ~ ~ . ~ ~ the LUMO is the degenerate E, set which Longuet-Higgins et al.46call the 4e(a) orbitals. These orbitals are shown in Figure 2, redrawn from those shown as the hybrids of the original pair of 4e(a) orbitals; the hybrids are appropriate for meso-substituted porphyrins. In 4-fold symmetric TPP derivatives such as 1 and 6 of Figure IC, the two 4e(a) vacant orbitals of Figure 2 are degenerate on the N M R time scale when N-H proton tautomerism is rapid (as it is at room t e m p e r a t ~ r e ~ ~However, ). for compounds 2,3,and 5 the molecular symmetry is lowered to 2-fold, and thus the degeneracy is expected to be lifted. This will cause one or the other of the 4e(a) orbitals to be favored for T conjugation. Orbital 2a should be favored for compounds 2 and 3,while 2b is favored for compound 5. Compound 4 would be expected to use one of the two original unhybridized 4e(a) o r b i t a l ~not ~ ~ shown , ~ ~ in Figure 2. Because of the 2-fold symmetric nature of the 4e(r) orbitals, placing one electron-donating substituent on the upper meso position (a)of Figure 2a (compound 2;XI = NEt,, X2-X4 = C1) should place electron density not only at meso position a (top) but also at the y position (bottom). Thus the pyrrole protons closest to meso position y should feel a stronger ring current than those closest to meso positions p and 6, and NMR spectroscopy should give direct evidence of the importance of a conjugation. The preparation and investigation of the NMR spectra of the six compounds of Figure IC and their Zn(I1) complexes are the subject of this report. Experimental Section The compounds of Figure IC were prepared by a variation of the Adler method:47 2 equiv of freshly distilled pyrrole (Eastman) and 1 equiv each of p-(diethy1amino)benzaldehyde and p-chlorobenzaldehyde (both Aldrich) were refluxed together in propionic acid for (45) Gouterman, M. J. Chem. Phys. 1959, 30, 1139-1161. Gouterman, M . In ‘The Porphyrins”; Dolphin, D., Ed.; Academic Press: New York, 1978; V O ~111, . pp 1-165. (46) Longuet-Higgins, H. C.; Rector, C. W.; Platt, J. R. J . Chem. Phys. 1950, 18, 1174-1181. (47) Adler, A . D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. J . Org. Chem. 1967, 32, 476.

30 min. The reaction mixture was allowed to cool for 1 day. No crystals were obtained from the solution, and thus the product mixture plus side products were isolated by evaporating off the propionic acid, dissolving the resulting tarry solid in dichloromethane, and shaking with 6 M N H 4 0 H to remove traces of propionic acid and to deprotonate the diethylamino groups. The organic layer was then passed through a dry silica gel column (Baker chromatographic grade) to remove tar and side products. The column was eluted with dichloromethane, and the fractions were evaporated to dryness. The isomers 1-6 were separated on a gravity silica gel column packed with use of petroleum ether as solvent and eluted with 70% benzene-30% petroleum ether solution. The mass spectrum of compound 1 had peaks due to M’, M+ - C6H4C1,M+ - 3C6H4C1,and the corresponding doubly charged ions; (diethy1amino)phenyl-containingisomers were not sufficiently volatile for mass spectra to be obtained. All isomers gave satisfactory elemental analyses, although they tended to hold water and/or benzene of crystallization. Anal. Calcd for 1 (analyzed as the dihydrate, C4HNN4Cl4O2): C, 67.0; H, 3.83; N, 7.10; CI, 18.0; 0, 4.1; N / C l , 0.395. Found: C, 66.4; H, 3.90; N , 7.37; C1, 18.4; 0 3.9 (by difference); N / C I 0.400. Calcd for 2 (analyzed as the monohydrate, benzene hemisolvate, CS1H41NSC130):C, 72.4; H, 4.88; N , 8.28; C1, 12.6; 0, 1.9; N/C1, 0.658. Found: C, 73.1; H, 4.84; N , 8.54; C1, 13.0; 0, 0.5 (by difference); N/C1, 0.658. Calcd for 4 (analyzed as the trihydrate, dibenzene solvate, C64H64N6C1203):C, 74.2; H, 6.23, N, 8.1 1; C1,6.84; 0, 4.6; N/Cl, 1.19. Found: C, 74.7; H, 7.20; N, 7.43; CI, 6.53; 0, 4.1 (by difference); N/C1, 1.14. Calcd for 5 (analyzed as the monohydrate, monobenzene solvate, C~~HS~N~C C,~77.7; O ) :H, 6.73; N , 10.2; CI, 3.70; 0,1.7; N/Cl, 2.76. Found: C, 78.2; H, 6.99; N, 9.75; C1, 3.58; 0, 1.5 (by difference); N/Cl, 2.72. The presence of varying amounts of benzene and water of crystallization in individual preparations of each isomer was confirmed by N M R spectroscopy: peaks at 7.35 and -1.3 ppm, respectively, could be decreased and eventually eliminated by repeated evaporation from CDC1,. The tetra-p-diethylamino isomer, compound 6, was in small yield and did not elute well with the above solvent mixture. Quantity preparation of it was camed out by the Adler method4’ and purification accomplished by chromatography on silica gel with benzene as solvent. Spectroscopic properties of 6 in pyridine were similar to those reported by Datta-Gupta et ala4* Anal. Calcd for 6 (analyzed as for the dihydrate, C6,H7,N8O,): C, 77.1; H , 7.54; N , 12.0, 0, 3.4. Found: C, 77.8; H, 7.60; N, 11.1; 0, 3.5 (by difference). Zinc was inserted into several of the compounds by an adaptation of literature methods:49 After the D M F solution cooled, an equal volume of dichloromethane was added and the mixture poured into a separatory funnel containing ca. 500 mL of HzO. The organic layer was drawn off without shaking and then reintroduced into the separatory funnel in the presence of a fresh 500 mL of H20. It was shaken and separated and this step repeated two or three additional times to remove traces of DMF. The zinc complexes were chromatographed on silica gel with benzene as solvent. Deuteriochloroform (Merck) was used as received. Several of the samples were evaporated from CDC13 before their N M R spectra were recorded to remove benzene of crystallization. Spectroscopic grade dichloromethane (Fisher) was used as solvent for UV-visible spectra. The identity of the separated compounds was confirmed by N M R spectroscopy on a Varian EM-360 and where possible by mass spectrometry on a Du Pont 490B. UV-visible spectra were recorded on a Cary 17 spectrophotometer. Further N M R investigations were carried out on the 250-MHz spectrometer in the chemistry department at the University of California, Berkeley, CA.

Results

Table I lists the UV-visible spectral data obtained for compounds 1-6. The UV-visible absorption maxima shift to longer wavelength as the number of diethylamino groups increases and will be discussed further in the Discussion section. Figure 3 shows the aromatic region of the N M R spectra of 1-6. The chemical shift and coupling data for P-pyrrole H, ortho and meta phenyl H, and N-H for 1-6, H2TPP, and several of its symmetrical derivatives and the Zn(I1) complexes (48) Datta-Gupta, N . ; Bardos, T. J. J . Heterocycl. Chem. 1966, 3,495-502. (49) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J . Inorg. Nucl. Chem. 1970, 32, 2443-2448.

Inorganic Chemistry, Vol. 21, No. 9, 1982 3345

Electronic Effects in Transition-Metal Porphyrins

Table I. Absorption Maxima and Extinction Coefficients‘ of Tetraphenylporphyrin Free Bases in Methylene Chloride compd

Soret shoulder

1 2 3

402 (16.08) 405 sh C

4

C

5 6

414 (34.89) 421 (8.00)

a Amax

IV

Soret 418 418 416 410 440 445

(63.682 (53.0) (42.43) (55.49) (24.84) (13.27)

in nm; E (in parentheses) in l o 3M - l cm-‘ .

514 516 518 514 521 530

(5.47) (5.45)b (5.36) (7.85) (6.30) (1.60)

548 553 568 559

111

I1

(2.45) 589 (3.22)b 589 (4.15) 588 (5.91) 583 5 7 3 (7.88) 585 (3.15)

(1.79) (2.32)b (2.90) (4.30)

Extinction coefficients estimated.

I 648 648 653 649 661 672

(1.89) (1.57)b (2.92) (3.09) (3.87) (1.61)

Not observed.

E *rl

M

i 8 0

8 5

1 0

7 s

7 0

I=

PP3

8 0

7 5

7 0 Ppn

30

8 0

7 5

Figure 3. 250-MHz NMR spectra of the aromatic regions of the H2TPP(p-Cl),(p-NEt2),series of compounds in CDCI, vs. Me& b, c, c’, d , and d’ are discussed in the text.

7 0

PPM

Symmetry

labels a ,

of 2, 4, and 5 are summarized in Table 11. Plots of the chemical shift data of Table I1 vs. the sum of the Hammett u constants ( E a ) of the phenyl substituents are shown in Figure 4. Discussion The electronic absorption spectra summarized in Table I for the six isomers of Figure IC are listed with adoption of the numbering system (I-IV) for the visible bands used by Falk.50 It has been shown by G o ~ t e r m a nthat ~ ~ the four visible bands of free-base porphyrins are due to the two possible transitions from AZuto E, porphyrin a orbitals each having a vibrational component. Bands I and I11 are ascribed to one transition (Q,) and its vibrational component, and I1 and IV to the other transition (Q,,) and its vibrational component, re~pectively.~~ The energies Q, and Q,, differ for the free-base porphyrins because the protons are fixed on two of the nitrogens during the time scale of the electronic absorption. The Soret band has been ascribed to the transition from AI, to E,,45 and here the x and y components of E, apparently do not lead to as large a separation in the energies of the two transitions. We observe “shoulders” on the Soret bands of four of the six isomers (not on 3 and 4, Table I), and in one case (compound 5), the intensities of “shoulder” and Soret bands are reversed. The (50) Falk,J. E. ‘Porphyrins and Metalloporphyrins”;Elsevier: New York, 1964; pp 72-80, 89-93.

cis (4) and trans (3) isomers of @-C1),@-NEt2)(TPP)H2 do not exhibit shoulders. The energy difference between the “shoulder” and the Soret bands is approximately one-third that between I and I11 or I1 and IV in compounds 1 and 2. The visible bands I-IV, as well as the Soret, shift generally to longer wavelength as p-chloro groups are replaced by pdiethylamino groups. Furthermore, the relative intensities of bands I and IV change through the series in such a manner that there is a fair inverse correlation between vI or vIv and q/qVfor compounds 1-6. Similar trends are observed for the Zn(I1) complexes of 1-6, which will be discussed e l s e ~ h e r e . ~ ~ Such correlations have been found previously for the a and @ bands of metallotetraphenylporphyrins as the electronic properties of phenyl substitued2 or electronegativities of axial ligand^^^,'^ are varied and have been correlated with changes in electron density in the porphyrin ring obtained from extended Huckel calculations on tin octaethylporphyrin dihalides (the halides are F, C1, Br, and I).54 Since band I of metal-free porphyrins is a component of what becomes the a band of metalloporphyrins, while band IV is a component of what becomes the @ band, it is not surprising that the same general (51) McDermott, G. A.; Walker, F. A,, manuscript in preparation. (52) Shroyer, A. L. W.; Lorberau, C.; Eaton, S. S.; Eaton, G . R. J. Org. Chem. 1980, 45,4296-4302. (53) Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1978, ZOO, 5075-5080. (54) Gouterman, M.; Shcwarz, F. P.; Smith, P. D.; Dolphin, D. J . Chem. Phys. 1973, 59, 676-90.

Walker, Balke, and McDermott

3346 Inorganic Chemistry, Vol. 21, No. 9, 1982 Table 11. Chemical Shifts of Tetraphenylporphyrin Protonsa compd

ob

H,@-NO,),TPP H,TPP H,@-CH,),TPP H,@-OCH,),TPP

3.112 0.000 -0.680 -1.072

pyrrole

o-phenyl

m-phenyl

N-H

8.806 8.860 8.869 8.862

8.657 (8.7) 8.230 (7.0, 1.8) 8.023 (7.7) 8.126 (8.4)

8.379 (8.7) 7.758‘ (7.0) 7.548 (7.7) 7.288 (8.4)

-2.95 -2.74 -2.78 -2.75

@-Cl),(p-NEt,),TPP compd

Xu

Ha

1 2

0.908 -0.15

9.039 (4.8)

3 4 5

-1.21 -1.21 -2.26

8.988 (4.8) 9.005 (4.8) 8.967 (4.8)

6 Zn-1 211-2

-3.32 0.908 -0.15

9.123 (4.0)

Zn-4 Zn-5

-1.21 -2 26

9.114 (4.0) 9.088 (4.8)

Zn-6

-3.32

9.059

Hd’.d

8.898 8.952

Isomers

Hb

Hc’,c

O-Hc1

8.796 (4.8)

8.825 8.804

8.763 (4.8) 8.765 (4.8) 8.737 (4.8)

8.780

8.1 14 (8.0) 8.1 15 [ 2 ] , 8.105 [ l ] (8.0) 8.145 (8.1) 8.134 (8.3) 8.145 (8.4)

8.915 8.885 (4.0) 9.107 9.084

8.872 (4.0) 8.859 (4.8)

8.932 8.891 8.879

8.132 (7.6) 8.123 [ 2 ] , 8.116 [ l ] (8.3) 8.137 (8.3) 8.154 (8.2)

o.HNEt,

m-Hcl

m-HNEt,

N-H

8.030 (8.3)

7.732 (8.0) 7.705 (8.0)

7.037 (8.3)

-2.86 -2.92

7.726 (8.1) 7.720 (8.3) 7.714 (8.4)

7.046 (8.5) 7.050 (8.6) 7.043 (8.5)

-2.73 -2.64 -2.56

7.037 (8.5)

-2.52

8.030 (8.5) 8.052 (8.6) 8.059 [ l ] , 8.045 [ 2 ] (8.5) 8.050 (8.5) 8.015 (8.0) 8.042 (8.7) 8.058 [ l ] , 8.046 121 . . (8.3) 8.067 (7.6)

7.740 (7.6) 7.721 (8.3)

7.039 (8.0)

7.718 (8.3) 7.712 (8.2)

7.047 (8.7) 7.045 (8.2) 7.061 (7.6)

Measured in CDCl,, with Me,Si as reference. Chemical shifts are in ppm, J coupling constants are in Hz, in parentheses, and relative intensities are shown in brackets. Reference 36. The u constant used for -NEt, is that of -NMe,. p-H at 7.769 ppm ( J = 7.0, 1.8 Hz). a

trends are observed for compounds 1-6. However, although this general trend is followed, the appearance of the spectra (A, e; Table I) classifies them as belonging to one of three subgroups of very similar spectra: (a) those containing mostly chloro substituents (compounds 1 and 2), (b) those containing equal numbers of chloro and diethylamino groups (compounds 3 and 4), and (c) those containing mostly diethylamino groups (5 and 6). (Note that bands I1 and I11 have coalesced in these compounds.) The two compounds in each subgroup are more similar to each other in terms of the appearance of the visible spectral bands I-IV than they are to the next member of the series. Thus it appears that the effects of molecular symmetry cannot be clearly and simply utilized in explaining the electronic absorption bands, and therefore the number of diethylamino vs. chloro groups is the major perturbing factor on the electronic absorption spectra of this series of compoundss5 This conclusion is quite different from that reached from study of the N M R spectra of isomers 1-6, now to be discussed. The aromatic regions of the 250-MHz N M R spectra of the six isomers of H2TPP(p-Cl),(p-NEt2), of Figure IC, numbered in order of their elution from the chromatography column, are shown in Figure 3. Also included in Figure 3 are symmetry labels for the types of pyrrole H. There are four possible types of pyrrole protons, based upon the identity of their nearest and next-nearest neighbors: Ha have p-(diethy1amino)phenyl groups as nearest and p-chlorophenyl groups as next-nearest meso neighbors, while H b have the reverse. H, have pchlorophenyl groups as both nearest and next-nearest meso neighbors, while Hd have p-(diethy1amino)phenyl groups as ( 5 5 ) It has been suggested (Gouterman, M., personal communication) that

the reason that the electronic absorption spectra of metal-free porphyrins and metalloporphyrins do not appear to respond to the lowered symmetry inherent in unsymmetrically substituted porphyrins such as those of the present study is that the absorption bands are too broad to allow such effects to be observed. This broadening is, in the tetraphenylporphyrins, presumably due to the wide distribution of angles between phenyl and porphyrin planes that are present at the instant of electronic excitation and thus the wide range of potential resonance (whether by K induction or r conjugation) interactions between porphyrin and phenyl rings on the time scale of electronic spectroscopy.

both types of meso neighbors. Further subclassification of c and d can be made if third-nearest meso neighbors are considered: we define type c as those that have a p-chlorophenyl group as third-nearest neighbor (six atoms away from the carbon carrying H,) and c’ as those that have a p-(diethylamino)phenyl in that position. Likewise, type d pyrrole protons are those which have a p(diethy1amino)phenyl group as the third-nearest meso neighbor, and d’ are those that have a p-chlorophenyl group in that position. The differentiation of c and c’ or d and d’ by N M R shift would be evidence of R conjugation, as suggested by Figure 2 and the discussion at the end of the Introduction. As it turns out, this subclassification of c, c‘ and d, d‘ is not necessary to explain the spectra of Figure 3, since their N M R shifts (c, c’ in 2 and d, d’ in 5) are identical (see discussion below). In a comparison of the pyrrole-H patterns and chemical shifts observed for the six compounds of Figure 3 and the chemical shift data for phenyl H and N-H presented in Table 11, several conclusions can be drawn regarding the effect of electron-donating and -withdrawing substituents on the chemical shifts and splitting patterns of the pyrrole protons, as well as phenyl H and N-H. Considering first the pyrrole protons, it is evident that para phenyl substituents in symmetrical tetrasubstituted HzTPPs cause a slight upfield pyrrole-H shift if the substituent is electron withdrawing (p-C1: u = +0.22736)(Figure 3A, Table 11) and a downfield pyrrole-H shift if the substituent is electron donating (p-NEt,: u = -0.8356) (Figure 3D, Table 11) with respect to H,TPP (p-H: u = 0.0036)(Table 11). In fact, for a series of tetra-para-substituted TPPs, both pyrrole-H and N-H resonance positions vary linearly with the sum of the Hammett u constants of the substituents (Figure 4). The total chemical shift difference between tetra-p-chloro and tetra-p-diethylamino derivatives of H2TPP is 0.090 ppm. The mixed phenyl-substituted isomers of Figure 3B,C,E,F have two pyrrole-H environments, on the basis of their observed chemical shifts, one (8.74-8.83 ppm) for those protons im( 5 6 ) The u constant for p-NEt, was assumed to be approximately equal to that for p-NMe2.I6

Inorganic Chemistry, Vol. 21, No. 9, 1982 3347

Electronic Effects in Transition-Metal Porphyrins

,

-24,

-3.0

1

1 I

I

t

L

I

87

B 80

-

1 01 I

-3 0

n.HNEt2

1

-20

-10

00

10

20

30

X U

Figure 4. Linear free energy relationship between (A) N-H proton chemical shift or (B) pyrrole-H and ortho phenyl-H chemical shift and the sum of the Hammett u constants (xu) of the phenyl substituents. Legend: filled circles, chemical shifts of symmetrically tetra-para-substituted H2TPPs;crosses, weighted average chemical shifts of pyrrole-H protons of unsymmetrically para-substituted H2TPPs; squares, observed chemical shifts of unsymmetrically para-substituted H2TPPs. Least-squares slopes ( p ) of the linear free energy relationships for pyrrole H and N-H are also given.

mediately next to meso positions carrying a p-chlorophenyl group (types b,c‘,c, Figure 3B,C,E,F) and one (8.92-9.04 ppm) for those immediately next to meso positions carrying a p(diethy1amino)phenyl group (types a,d’,d, Figure 3B,C,E,F). The weighted average chemical shift of the pyrrole H of 2-5 lies on the same line as do the tetrasubstituted H2TPPs discussed above (Figure 4), but the individual pyrrole H resonances of types b,c,c’ and a,d,d‘ vary more in the opposite direction than does the average pyrrole-H resonance, when plotted against E o (Figure 4). The two lines b,c,c‘ and a,d,d’ are nearly parallel, with an average separation of ca. 0.2 ppm. This separation we will call the “nearest-neighbor substituent effect”. Our interpretation and explanation of the opposite slopes of the two individual lines b,c,c‘ and a,d,d’ will be discussed below. Pyrrole rings in which one proton is near a p-chlorophenyl (Hb) and the other is near a p-(diethy1amino)phenyl (Ha) have their proton resonances split into an AB quartet, which is reduced almost to first order at 250 MHz (J = 4.8 Hz). The two nearly first-order doublets are centered slightly “outside” the resonance positions of the unsplit signals of Hc,dand Hd,d/ in isomers 2, 4, and 5, with Ha being centered 0.015 ppm downfield from in 4 and 5 and Hb centered 0.008-0.01 5 ppm upfield from HC,& in 2 and 4 (see Figure 4). Thus Ha and Hb feel a slight (0.008-0.01 5-ppm) effect from the electron-donating or -withdrawing effect of their next-nearest

neighbor. No splitting of H,and H d or Hd and Hd’is observed at 250 MHz for compounds 2 and 5, respectively, and therefore the electronic effect of the third-nearest meso neighbor is unmeasurably small at 250 MHz. Thus there is no evidence of H conjugation as suggested by Figure 2 and the last paragraph of the Introduction. Rather, the electronic effects of substituents on the meso phenyl rings on the pyrrole proton environments of the porphyrin attenuate rapidly, with the effect at Hb being less than a tenth of that at H, in 2. No evidence exists in the pyrrole-H N M R patterns of Figure 3 of H delocalization of these effects to other pyrrole-H positions of the porphyrin ring. Further evidence for the lack of electron delocalization by direct a conjugation is found in the ortho phenyl-H resonances of compounds 2 and 5. All ortho H in tetraphenylporphyrins and their diamagnetic metal complexes are subject to a deshielding effect (downfield shift) as a result of the porphyrin ring current, because the ortho phenyl protons lie outside the perimeter of the ring.34 Because the expected electron density in the 4e(a) orbitals of Figure 2 is largest at the meso positions, placement of one electron-donating phenyl substituent at meso position a (compound 2) should increase the electron density at the opposite (trans) meso position (y) as compared to that at the cis ( p and 6) meso positions. Thus the ortho phenyl H of the trans (y) p-chlorophenyl group should be shifted further downfield than those of the cis ( p and 6) p-chlorophenyl groups if K conjugation is important. Indeed, close scrutiny of the o-Ha doublets of 2 shows that two resonances do occur, of relative intensities 1:2. However, the o-Ha doublet of relative intensity 1 (y) is slightly upfield of that of relative intensity 2 (@and a), indicating that the trans- (y) p-chlorophenyl group is subject to a weaker ring current than are the cis- (0 and 6) p-chlorophenyl groups. Since this is in the opposite direction from that predicted above for direct a conjugation, the o-H resonance of the tram-p-chlorophenyl group of 2 (and likewise the o-H resonance of the trans-p-(diethy1amino)phenyl group of 5) provides further evidence of the lack of direct H conjugation. Instead, we see that the larger ring current at the cis (p and a) meso positions, as evidenced by the slightly downfield positioning of the o-H peaks of the cis-p-chlorophenyl groups (Figures 3 and 4), is consistent with the conclusion reached above that the inductive effect felt at the pyrrole position closest to the unique substituent drops off rapidly as one progresses away from that meso position: Enough inductive effect reaches as far as the cis meso positions so as to make them slightly more electron rich than the trans meso position. In fact, some electron density may actually be “stolen” from the trans meso position, as discussed in the next paragraph. Rather than direct a conjugation, the N M R shifts of the pyrrole protons (as well as the ortho phenyl H ) of 1-6, as summarized by Figure 4, give evidence of the importance of H induction: With a focus first on the pyrrole H , those adjacent to p-NEt2-bearing phenyl groups are shifted away from the average line as the number of p-C1-bearing phenyl groups increases (a,d,d’ line). Likewise, pyrrole H adjacent to pC1-bearing phenyl groups shift away from the average line as the number of p-NEt2-bearing phenyl groups increases (b,c,c’ line). Thus, considering the effect from the p-NEt, point of view, the meso positions bearing the p-NEt, substituents appear to ”rob” a-electron density (ring current) from distant parts of the a system to build up those regions closest to the meso-p(diethy1amino)phenyl substituents. Thus, for compound 2, Ha feels a large ring current (larger than it should, according to the “average” behavior), while Hb and Hc,d have been “robbed”: Hb, which is in the same pyrrole ring as Ha, has been robbed of slightly more electron density than Hc,c,. Proceeding through the series, we see the same behavior, scaled to the relative number of diethylamino and chloro groups, of

3348 Inorganic Chemistry, Vol. 21, No. 9, 1982 compounds 3, 4, and 5. For 5, Ha still feels a larger ring current than it should, according to the behavior of 6, 1, and the other symmetrical H2TPPs of Table I1 and Figure 4, but here it is joined by Hd,dt, so that the amount of electron density “robbed” from the H, carbons is only about one-third of what it was in compound 2. Note, however, that Ha, which is adjacent to H b in the same pyrrole ring, still has a slightly greater downfield (ring current) shift than Hddt,thus indicating that the two meso positions adjacent to Ha are able to “rob” that meso position adjacent to Hb slightly better than the meso position opposite the p-chlorophenyl group. The same explanation accounts for the fact that the ortho H of the opposite (trans) phenyl group in 2 appears to have been “robbed” of a small amount of ring current and that of 5 appears to have slightly more than its /3 and 6 neighbors. The proton NMR spectra of compounds 1-6 appear to allow distinction between the two possible modes of transfer of resonance effects from phenyl substituents to the porphyrin ring which could not be distinguished in the Ni(I1) and V(IV) TPP axial ligation studies:” Moderate conjugation must be ruled out on the basis of the pyrrole-H chemical shifts observed in 1-6, leaving A induction as the major means of transfer of the resonance capability of X, when placed on a phenyl ring, into the porphyrin plane. In addition, 0 induction is probably involved, as suggested from the Ni(I1) and V(1V) ligation studies.” The effect of substituting a diamagnetic metal (Zn2+)for the two N-H protons on the patterns and chemical shifts of the protons of 1-6 is very small (Table 11): The pyrrole-H protons are shifted -0.09-0.13 ppm downfield, but the Ha,, coupling constant remains the same, and €-Ic,?and Hd,dteach remain equivalent. Linear relationships such as those shown in Figure 4 exist for Zn(I1) complexes of 1-6. Little effect on the phenyl protons is observed. Thus, the conclusions reached above concerning the importance of A induction and lack of importance of T conjugation appear to hold equally well for metal-free and diamagnetic metallotetraphenylporphyrins. (The effect of this A induction on the N M R spectra of paramagnetic metallotetraphenylporphyrins is, however, dramatic, as is discussed e l ~ e w h e r e . ~ ~ ) The effect of phenyl substitution on the N-H proton shifts of metal-free TPPs (Table 11, Figure 4) provides further evidence for the lack of T conjugation between substituent X and the porphyrin ring. The N-H protons, which are inside the perimeter of the ring and thus experience a strong upfield ring current shift,27,32,57 should be indicative of changes in the (57) Abraham, R. J. Mol. Phys. 1961, 4, 145-152.

Walker, Balke, and McDermott average strength of the ring current, since they exchange rapidly among the four porphyrin nitrogens at ambient temp e r a t u r e ~ .The ~ ~ variation in N-H chemical shift is linear for symmetrical H2TPPs, as well as for most of the unsymmetrical H2TPPs of this series, and has the same sign slope (negative) as the line for the average pyrrole-H shifts for the same series of compounds. If the variation in N-H chemical shift were a ring current effect only, its slope should be opposite in sign from that of the pyrrole H. Since it is not, and since its magnitude is so much greater, it appears that the variation in N-H chemical shift among this series of compounds is overwhelmingly controlled by basicity effects (c bonding). In this light, it may be that the deviation of compound 2 from the N-H linear free energy line is related to the unsymmetrical distribution of electron density at the porphyrin nitrogens of 2 which may stabilize the tautomer that has the two protons on adjacent rather than o p p o ~ i t enitrogens. ~ ~ ? ~ ~ That 5 does not deviate significantly from the linear free energy correlation line may suggest more rapid proton tautomerism for 5 than for 2. These possibilities are under further investigation. Finally, the question might be raised as to whether the pyrrole protons are affected by changes in the ring current of the nearby phenyl groups caused by changes in para substituent, since the pyrrole-H protons are, on the average, situated inside the perimeter of the adjacent meso phenyl ring and thus potentially susceptible to an upfield shift due to the shielding ring current effect of the phenylss From the data presented in Figures 3 and 4 and Table I1 it appears that this is, at most, a minor factor affecting the chemical shift of the pyrrole protons, since the pyrrole-H protons adjacent to p-(diethylamino)phenyl groups are shifted downfield (Le,, responding to their being outside the perimeter of the porphyrin ring current loop, which we view as being particularly electron rich at the meso positions to which p-(diethy1amino)phenyl groups are attached), rather than upfield, as they would be if they were responding to being inside the ring of the electron-rich p-(diethy1amino)phenyl group. Acknowledgment. The authors wish to thank Dr. Yitmin Liang for running the 250-MHz N M R spectra at the University of California, Berkeley, CA. The financial support of the N S F (Grant No. CHE-79-18217) and N I H RCDA (Grant No. 5 KO4 G M 00227) is gratefully acknowledged. Registry No. 1,22112-77-2; Zn-1, 291 16-33-4;2, 82044-64-2; Zn-2, 82044-65-3; 3, 82044-66-4; 4, 82044-67-5; Zn-4, 82044-68-6; 5, 82044-69-7; Zn-5, 82044-70-0; 6, 14609-52-0; Zn-6, 82044-7 1- 1. (58) Abraham, R. J.; Jackson, A. H.; Kenner, G. W.; Warburton, D. J . Chem. SOC.1963, 853-862.